1. Field of the Invention
The present invention relates to a method and apparatus for masking a part of video images taken with a surveillance camera for the purpose of privacy protection.
2. Description of the Related Art
A general surveillance camera can be panned or tilted by a pan/filter for changing the surveillance area. FIG. 1 shows panning and tilting operations of a surveillance camera according to the related art. A surveillance camera equipped with pan/tilt/zoom capabilities, which is generally referred to as the PTZ surveillance camera, comprises a camera unit 1a and a pan/tilter 1b for rotating the camera unit 1a horizontally or vertically. As shown, the pan/tilter 1b is attached to a place (e.g., the ceiling of a particular room) and the camera unit 1a is connected to the pan/tilter 1b. 
The pan/tilter 1b can change the surveillance area of the camera unit 1a by tilting and rotating the camera unit 1a vertically or horizontally. If the surveillance camera is equipped with a zoom lens, the video image of an object under surveillance can be magnified or reduced by adjusting the zoom lens.
The surveillance camera may be equipped with privacy protection capability, which can mask a part of the video image taken from a surveillance area if the video image contains particular objects which require privacy protection such as a bathroom window, a gate to a particular place, etc. The size and position of the video block for masking is set up initially by the operator of the surveillance camera. The surveillance camera does not output the video image corresponding to the masked video block, thereby preventing a part of the video image from being viewed or recorded.
FIG. 2 is an example of the masking operation according to the related art. If a masking block 2a, which is specified by its edge coordinates (x0,y0) and (x1,y1) on the video frame, is set and a video image shown in FIG. 2B is taken from a surveillance area, the video image corresponding to the masking block 2a is made to be black or gray. As a result, the video image as shown in FIG. 2C is outputted by the surveillance camera.
As mentioned above, the surveillance area can be changed by the pan/tilter of the surveillance camera. Due to mechanical errors, however, the actual rotational displacement may not be the same as the intended pan/tilt angles. Further, such mechanical errors make the masking block 2a deviate from the initially set (actual) privacy protection area 2b as shown in FIG. 2D, if the camera is panned and/or tilted from the surveillance area in which the masking block for the privacy protection area is first set up and then returns to the original surveillance area. In this case, images taken from a part of the privacy protection area 2b may be exposed and recorded, which is a problem.
Moreover, even in the case of surveillance cameras not equipped with pan/tilt capabilities, the actual surveillance area may deviate slightly from the originally set surveillance area due to operating environments (e.g., wind or vibration). In this case, the privacy protection area cannot be masked accurately or completely by the masking block like the previous case and thus the same problem where a part of the privacy protection area is exposed occurs.